What are entropy perturbations?

I am self studying cosmology with the help of some text books ( books by Dodelson, Mukhanov, ...) and somewhat struggling to figure out what it is really mean by entropy perturbations. How is it different from adiabatic perturbations. Any help to get a better intuitive understanding is greatly appreciated. Is there any reference it is better explained? Thanks -Lalith

I can only offer my own thoughts based on some thermodynasmics, but the following seems very reasonable to me:

The class of pertubation you allready know is the adiabatic one. IF we assume reversibility, adiabatic processes are exactly isentropic (fixed entropy) ones. It makes therefore sense to divide the perturbations into two classes:

adiabatic (isentropic)
other (non isentropic) where entropy changes.

Furthermore, as δQ = TdS (for the reversible heat transfere δQ), every heat tranfere has to involve either temperature or entropy change. It makes therefore sense to decompose every non vanishing perturbation into:

Thanks for the reply. I did further reading and think now have better understanding. Recent book(2) by Gorbunov and Rubakov has a good explanation (book 2 page 62). Entropy (also called isocurvature) perturbations happens when the medium has many components (radiation, dark matter baryons, etc), and the composition become spatially inhomogeneous. Energy density of eack component cloud vary but the but the sum of all the components stay unchanged. So no energy density fluctuations (so isocurvature) although the composition (thus entropy ) changes.